Under European law, switching on the high beam in continuously illuminated areas is prohibited. When received light has an alternating current (AC) component or a certain frequency component, it may be concluded that roadway illumination is present. However, this requires a high scanning rate of the optical sensor for recognizing roadway illumination units of this type, which is only possible using a small image sensor. This approach is problematic not only due to the high scanning rate for camera systems, but also due to the increase in clocked light sources in automobiles. For example, LEDs are controlled by pulse-width modulation (PWM). “Chopping” and transforming, or clocking of the voltage, for example, may be used for stabilizing halogen light sources or for generating high voltage for xenon light sources.
Alternatively, the continuous illumination may be recognized by detecting urban areas/cities. For this purpose, speed limits (50 km/h, for example) may be used. Another option is to use map/navigation system information that indicates where a city begins; however, map material quickly becomes outdated for technical reasons. The recognition of cities does not correspond to the recognition of continuous roadway illumination, since intra-urban illuminated thruways or continuously illuminated freeways (such as in Belgium) are not recognized in this way. Switching on the high beam is prohibited on these roadways as well, even if they are not located in a city.
German Patent Application No. DE 10 2009 028 342 A1 describes a method for determining whether an area is illuminated. The method evaluates image data of a camera. The area is assessed as being illuminated if at least two stationary roadway illumination units are recognized based on the image data, and if these roadway illumination units have a spatial distance from one another.